Quickly align edge loop?
For the edge loop in your picture, with the Pivot Point  set to Median Point, Individual Origins, or Bounding Box Center, you can press SY 0.

That will scale all the selected vertices along the Y axis to 0, meaning all together, because the scaling is constrained to the Y axis, you will get a straight line perpendicular to the Y axis.

SX 0 for the X axis
SY 0 for the Y axis
SZ 0 for the Z axis